Thanks to Fibers and Geometry — Imaging

Some researchers believe that people would benefit from more fiber — and not just in their diets. In the June issue of Nature Materials, a group at MIT in Cambridge, Mass., has demonstrated fiber-based imaging that is free of conventional optics and detectors, offering such advantages as the ability to determine the direction of photons arriving from anywhere and the ability to build very large imagers.

A scanning electron microscopic cross section of a photodetecting fiber shows the photoconductive core, the electrodes that run along the length (the squares that protrude from the core) and the protective polymer that encases everything. The fibers form the basis of lens-free photodetecting geometric constructs. Courtesy of Yoel Fink’s group, MIT.

The key to the technique is a specialized fiber developed by the group. Metal electrodes run along the length of a photoconductive glass core, and a transparent and tough protective polymer surrounds both. The fiber acts as a one-dimensional photodetector, responding to light by generating an electrical signal.

With this fiber, the scientists created structures that acquired images without optics. They constructed a globe, placing the fiber along the lines of latitude and longitude. Photons striking the fibers from any direction generated a signal upon entry and exit and, from these two points, the researchers determined the beam’s directionality.

MIT scientists constructed a sphere out of photodetecting fibers. When light traversed the globe, the fibers detected its entry and exit, enabling the researchers to record beams coming from any direction. Courtesy of Greg Hren, Research Lab of Electronics, MIT.

In a more sophisticated application, they created a planar array with the fiber and used algorithms developed for computed tomography to measure intensity. Two such arrays — capturing amplitude and phase — enabled object image extraction without the presence of a lens.

Such fiber arrays could be extremely large yet lightweight, permitting various applications. For example, a 15 × 15-m-array composed of 100-μm-diameter fibers spaced 1 cm apart would have a photosensitive area of 4.5 sq m and yet weigh only 700 g, or 1.5 lb. The construct would image while being essentially transparent. “This may enable large-aperture and lightweight space telescopes,” said Yoel Fink, research team leader and associate professor of materials science.